This invention relates to a nickel base alloy structure provided with an aluminum coating and methods of producing same, and more particularly it is concerned with a blade and a nozzle of a gas turbine both of which are made of a nickel base alloy and are provided with an NiAl coating on its surface having excellent corrosion and thermal shock resistant properties and a method of producing same.
Nickel base alloys are often used for forming parts of gas turbines, jet engines, etc., that are exposed to heat of high temperature. Particularly, the blade and nozzle of a gas turbine are often exposed to corrosion and oxidation inducing combustion gas atmosphere containing sulfur, chlorine, vanadium, sodium and lead etc. Thus the nickel alloys are required to have not only high mechanical strength at elevated temperatures but also corrosion and oxidation resistant properties against combustion gas. To impart corrosion and oxidation resistant properties to the nickel alloys, it is necessary to increase the contents of chromium, aluminum and other alloying elements in the alloys. However, an increase in the contents of chromium, aluminum and other alloying elements in excess of acceptable levels would cause a reduction in the ductility of the nickel alloys and other defects, with the result that there are naturally limits to the contents of these alloying elements that could be added to the nickel alloys.
Under these circumstances, it has hitherto been common practice to protect the nickel alloys from combustion gas by applying surface treatment to the alloy, such as the provision of a coating layer on their surfaces. Typical treatment for applying coating on the nickel alloys is an aluminum coating applying treatment, in which Al coating is applied by such various means as the immersion of an alloy in a molten salt, sputtering, vaporization deposition, a CVD process and a pack cementation process. In these methods, the pack cementation process is most popular which, as described in Japanese Patent Publication No. 3729/1973 (corresponding to U.S. Ser. No. 796,906 of Feb. 5, 1969), includes the steps of embedding a nickel base alloy in a powder mixture of aluminum powder, inert refractory material powder (alumina, for example) and halogenated active agent powder (NH.sub.4 Cl, NH.sub.4 F, NaCl, NaF, AlCl.sub.3, etc.) and maintaining the alloy at a predetermined high temperature to cause the aluminum to be diffused into the nickel alloy to form on its surface a coating high in aluminum content. The coating applied to the surface of a nickel base alloy by the pack cementation method is composed of intermetallic compounds, such as Ni.sub.2 Al.sub.3, NiAl, etc., formed by the combination of aluminum and the nickel in the nickel base alloy that is treated. When the treatment by the pack cementation method is carried out at low temperature below 850.degree. C. to specifically increase the corrosion resistant property of the alloy by increasing the amount of chromium in the coating layer, the coating layer becomes composed mainly of Ni.sub.2 Al.sub.3 low in mechanical strength. This makes it necessary to apply heat treatment to the alloy following the pack cementation treatment so as to change the coating to another coating composed mainly of NiAl having increased ductility. After such heat treatment, the coating is composed of a first layer of NiAl rich in aluminum and a second layer of NiAl rich in nickel. The prior art pack cementation method has suffered the disadvantages that, since the heat treatment for changing the composition of the coating has been carried out at high temperature for a prolonged time, the layer of NiAl rich in nickel accounts for the major part of the coating, with the result that the coating provided is not necessarily high in corrosion resistance.